The effect of the hip flexors iliacus and psoas on rotation of the femur is significant in understanding structural patterns and developing effective bodywork strategies.
Robert Schleip’s lecture notes (Schleip, 1988) suggests that hip flexors medially rotate the femur. Schleip focuses on the psoas primarily, but his argument is equally true for iliacus as well since they both attach at the lesser trochanter. Plate 466 in Netter’s Atlas of Human Anatomy (Netter, 1989) indicates that the hip flexors laterally rotate the femur.
I disagree with Netter because it does not take the location of the mechanical axis into account as Schleip does. I agree with Schleip that to the extent that the flexors rotate the femur at all, they are medial rotators. In contrast to both Netter and Schleip, I propose here that the hip flexors are “designed” to be rotation-neutral.
Figure 1 reviews the role of the psoas and iliacus as hip flexors. The anterior force of the flexors acting on the lesser trochanter flexes the hip. The path of the flexors over the ilium requires them to traverse posteriorly to reach the lesser trochanter, and thus provide an anterior force. Once the hip has been significantly flexed, the path of the flexors is not interrupted by the ilium and a forward force is no longer needed to continue flexing the femur as simple lift continues flexion.
The arrows show the component forces acting on the lesser trochanter. There are both anterior and superior forces. The ?x? indicates the location of the lesser trochanter on the medial side of the femur.
Illustration adapted from Anatomy of Movement by Blandine Calais-Germain, with permission of Eastland Press, PO Box 99749, Seattle, WA 98199. Copyright 1993. All rights reserved.
How can we evaluate the rotational effect of these flexors? In Figure 2 the horizontal axis of rotation for hip flexion is shown passing through the hip joint. In this arrangement, a forward pull on the lesser trochanter will bring the knee forward, flexing the hip, because the lesser trochanter is below the axis of rotation.
Here the ?x? marks the rotational center of the hip joint and the horizontal line the horizontal axis of rotation for the hip.
Forward pull on the lesser trocahnter below the axis will yield hip flexion.
We can apply the same analysis to rotation. We can locate the vertical axis of rotation (VAR) and then observe on which side of this axis the lesser trochanter lies. If it is lateral to the VAR, the hip flexors are medial rotators; if medial then they are lateral rotators. It is also important to note that the closer VAR is to the lesser trochanter, the less strongly it will tend to rotate the femur.
But how do we place the axis? In the example above, we used two points to establish the axis: the left and right hip joints. We need two points to establish the VAR as well. We can use the hip joint for one of the VAR points, but where is the other?
Schleip draws his VAR to pass through the center of the tibia, as shown in Figure 3. This conventional VAR yields a lateral placement of thethe lesser trochanter and thus a medial rotation effect of the hip flexors.
The VAR as placed by Schleip. In this placement the hip flexors would have a sight medial rotetion effect.
But is this the correct place for the VAR? In a hinge joint such as the elbow, the axis of rotation is determined by the shape of the bones in the joint. The hip however is a true ball and socket joint, and is not constrained by joint morphology. We must look for another constraint. If the foot were fixed on a point around which it could rotate, (and the knee was straight) this would serve the purpose of our second point and we could draw the VAR through the hip and the fixed point. However, this is not the case, and the knee is often bent. Physics tells us that in the absence of a second point, the VAR will pass through the center of gravity of the leg.
The determination of the exact center of gravity is relatively complex.
Further, it is different for each specific leg, and varies with the degree of bend and rotation in the knee and foot. We can arrive at a close approximation by drawing a line which passes through the hip joint and which divides the mass of the leg equally on each side. We must account for the varying densities of bone, fat, and myofascia.
I prefer to draw the line more lateral than Schleip, as the line through the center of the tibia seems to leave an excess of mass lateral to it. Without a careful analysis for each particular situation, the exact location is unknown.
Figure 4 shows a more lateral VAR, perhaps coming closer to bisecting the mass of the leg, and brings the VAR closer to the lesser trochanter (see image below). This in turn reduces the rotational effect of the hip flexors. While the change in location may seem small, note that if distance between the VAR and the lesser trochanter changes from 1 cm to 0.5 cm, the rotational force will be reduced by half.
The VAR as placed by Schleip (black), and a more lateral VAR (gray). The VAR is defined by 2 points: the rotational center of the hip joint and the center of gravity of the leg. I suggest that the center of gravity of the leg is nore lateral than the line drawn by Schleip.
Leaving this line of thought aside and looking at the question from another perspective, we can ask a more general question about the “design” of the femur. Why is it shaped like a “7” in the first place? It is an unwieldy design for load bearing and requires much more bone than a simple vertical shaft. However, the “7” shape has at least one outstanding advantage: it gets a lever out away from the VAR which allows the leg to be efficiently rotated, an essential function in negotiating the terrain we encounter and in changing direction’.
The farther away from the VAR a structure can attach, the more efficiently it can act to rotate the leg. A major “purpose”‘ then, of the trochanter, and the “7” shape of the femur, is to provide a lever for rotating the leg.
The lesser trochanter, by comparison, is reaching in the opposite direction, towards the VAR, diminishing its effectiveness as a rotator with every millimeter closer it gets.
I speculate that this reflects an ideal where flexing the hip does NOT interfere with the rotational positioning of the leg. Hip flexors have to be big to do their job. Rotators don’t have to be as big. Rotators are orienting the leg, not doing the big work of moving and accelerating the leg through space like the flexors.
In this view, the shape of the upper femur reflects two contrasting objectives: the ability to accurately and efficiently orient the leg in rotation, and to simultaneously flex the hip with strength and not overwhelm the smaller rotators. It would appear to be a more efficient design to have the hip flexors working directly on the VAR, and not require the smaller rotators to counter any rotational effects of the flexors with each stride.
Thus we see that the greater trochanter reaches away from the VAR to afford efficient rotational orientation of the leg, and the lesser trochanter reaches in towards the VAR to allow hip flexion without countering any of the orienting work of the rotators.
Is there in reality some medial rotational effect of the hip flexors? Certainly we could find an individual for whom this was true. But does this happen by “design”? Perhaps there is some lateral rotation force generated by another structure which requires the flexors to provide a small medial rotation force to maintain rotational equilibrium. While possible, this also would seem to be a less than efficient design, requiring muscular effort which could be avoided by placing the lesser trochanter on the VAR. An individual with such an inefficient design would tire before a contemporary whose lesser trochanter was placed for more efficient movement, and might arrive late for food and mate.
I suggest that the hip flexors are essentially rotation-neutral.
Calais-Germain, Blandine, 1993. Anatomy of Movement. Eastland Press, Seattle.
Netter, Frank H., 1989. Atlas of Human Anatomy. Ciba-Geigy Corp., Summit, New Jersey.
Schleip, Robert, “Lecture Notes on Psoas and Adductors,” Rolf Lines, Vol. 25, No. 5, Page 19
1 The shape of the trochanter has another significant advantage in that it allows the vertical fibers of the abductors to keep the pelvis more or less horizontal when we have more weight on one leg than the other.
2 I use quotes around “purpose” because evolutionarily there is no purpose. The reason our trochanters have the shape they do is because more of our ancestors with this shape survived to reproduce than any other, and because trochanter shape is an inherited trait.